The invention relates generally to the field of optical measurements and measuring systems, and more particularly to a method and system for optical spectrum analysis that utilizes optical heterodyne detection.
Dense wavelength division multiplexing (DWDM) requires optical spectrum analyzers (OSAs) that have higher spectral resolution than is typically available with current OSAs. For example, grating based OSAs and autocorrelation based OSAs encounter mechanical constraints, such as constraints on beam size and the scanning of optical path lengths, which limit the degree of resolution that can be obtained. As an alternative to grating based and autocorrelation based OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems.
Optical heterodyne detection systems are utilized for optical spectrum analysis of an input optical signal. FIG. 1 is a depiction of a prior art heterodyne-based detection system that includes an optical coupler 110 that combines an input signal 102 from an input fiber 104 with a swept local oscillator signal 106 from a local oscillator source 105 via local oscillator fiber 108. The combined optical signal travels on an output fiber 118 and is detected by a heterodyne receiver 112. The heterodyne receiver converts optical radiation from the combined optical signal into an electrical signal. Square law detection results in mixing of the two combined optical signals and produces a heterodyne beat signal at a frequency that is equal to the frequency difference between the combined optical signals. The heterodyne beat signal is processed by a signal processor 116 to determine a characteristic of the input signal, such as frequency, wavelength, or amplitude. The resolution of heterodyne-base OSAs is directly related to the bandwidth of the heterodyne receiver. Specifically, the wider the bandwidth of the heterodyne receiver, the better the amplitude resolution of the OSA and the narrower the bandwidth of the heterodyne receiver, the better the frequency resolution of the OSA. Furthermore, the amplitude accuracy of the heterodyne receiver depends on the sweep rate of the local oscillator signal. For example, to achieve an industry acceptable amplitude accuracy of approximately 0.1 dB given a local oscillator sweep rate in the tens of nanometers per second, the required heterodyne receiver bandwidth is in the tens of Megahertz. The bandwidth of the receiver defines the resolution of the heterodyne-based OSA when using a signal processing technique such as envelope detection. In envelope detection, the envelope can be approximated by an average, which is found by moving a window averaging filter across the detected heterodyne beat signal. FIG. 2 depicts envelope detection of a quadratic phase signal 120, such as a heterodyne beat signal that is observed from the mixing of highly coherent optical signals. The detected heterodyne beat signal is squared 122 and then filtered by window averaging to produce the detected envelope 126. As shown in FIG. 2, the resolution of the detected envelope is determined by the bandwidth of the heterodyne receiver. Although envelope detection works well in heterodyne-based OSAs, as the density of DWDM channels increases, there is a need for OSAs with greater resolution.
A method and system for heterodyne-based optical spectrum analysis involves filtering a heterodyne beat signal with at least one matched filter in order to improve the signal-to-noise ratio and the spectral resolution of the heterodyne-based optical spectrum analysis. In an embodiment, an input signal is combined with a swept local oscillator signal in an optical coupler. The combined optical signal is output to a receiver and a heterodyne beat signal is generated. The heterodyne beat signal is filtered by a matched filter unit and the filtered heterodyne beat signal is utilized to generate an output signal that is indicative of an optical parameter of the input signal. For highly coherent lasers the heterodyne beat signal exhibits a quadratic phase behavior. By utilizing the quadratic phase behavior of the heterodyne beat signal in signal processing, the resolution of a heterodyne-based OSA can be substantially improved over known heterodyne-based OSAs.
In an embodiment, the matched filter unit includes two matched filters that are orthogonal. The two orthogonal matched filters enable the output of the OSA to be independent of the phase difference between the input signal and the swept local oscillator signal. Specifically, using orthogonal matched filters allows for the spectral characterization of highly coherent, narrow-linewidth lasers whose spectral features are narrower than the bandwidth of the heterodyne receiver.
A method for optical spectrum analysis that utilizes optical heterodyne detection involves providing an input signal, providing a swept local oscillator signal, combining the input signal with the swept local oscillator signal to create a combined optical signal, detecting the combined optical signal to generate a heterodyne beat signal, filtering the heterodyne beat signal with a matched filter, and generating an output signal from the filtered heterodyne beat signal that is indicative of an optical parameter of the input signal.
In an embodiment, the heterodyne beat signal is split into a first heterodyne beat signal and a second heterodyne beat signal. The first heterodyne beat signal is filtered with a first matched filter to produce a filtered first heterodyne beat signal and the second heterodyne beat signal is filtered with a second matched filter to produce a filtered second heterodyne beat signal. The output signal is generated from the filtered first and second heterodyne beat signal. In a further embodiment, the first and second matched filters are orthogonal.
In an embodiment, the filtering is performed in the time domain and in another embodiment, the filtering is performed in the frequency domain.
A system for optical spectrum analysis includes an optical coupler, a heterodyne receiver, and a signal processor. The optical coupler has a first input, a second input, and an output. The first input being optically connected to receive an input signal, the second input being optically connected to receive a swept local oscillator signal, and the output being optically connected to output a combined optical signal that includes the input signal and the swept local oscillator signal. The heterodyne receiver has an input for receiving the combined optical signal from the optical coupler and an output for outputting a heterodyne beat signal that is representative of the combined optical signal. The signal processor receives the heterodyne beat signal from the optical receiver and generates an output signal that is indicative of an optical parameter of the input signal. The signal processor also includes a matched filter unit for filtering the heterodyne beat signal before the output signal is generated.
In an embodiment, the matched filter unit includes a splitter for splitting the heterodyne beat signal into a first heterodyne beat signal and a second heterodyne beat signal, a first matched filter configured to filter the first heterodyne beat signal and to output a filtered first heterodyne beat signal, and a second matched filter configured to filter the second heterodyne beat signal and to output a filtered second heterodyne beat signal. In a further embodiment, the first and second matched filters are orthogonal. In a further embodiment, the first and second matched filters are configured to filter in the time domain. In a further embodiment, the first and second matched filters are configured to filter in the frequency domain.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.